A magneto-resistive (MR) head, as used, for example, for reading magnetic tapes, generally comprises a thin film of a material whose resistance varies with the magnitude and direction of an external magnetic flux (such as generated by data on a magnetic tape) applied to the head. As with other types of sensors, the signals read by an MR head must be amplified. However, because of a non-linear relationship between magnitude of the external flux and the resulting resistance of the head, the head must be biased to maintain circuit operation within a linear portion of the characteristic curves of the head and the amplifier.
Older MR heads have three terminals: two outer signal terminals to provide the signals to be amplified and a center terminal connected to a ground potential to provide symmetrical biasing of the head and impedance matching of the two outer terminals. More recently, efforts to increase the amount of data which can be stored on a tape by increasing the bit density of each track and decreasing the track width to increase the number of tracks on the tape have made it necessary to reduce the size of the tape head to such an extent that the center terminal has been eliminated.
Biasing can be achieved by providing a constant current through the head to create a constant voltage potential across the two signal terminals of the head. The varying magnetic flux from the tape generates a high frequency varying voltage across the head on top of the constant bias voltage. Alternatively, a constant voltage can be provided across the head which creates a constant current through the head; the varying magnetic flux from the tape generates a high frequency varying current through the head on top of the constant bias current. With either method, the high frequency varying voltage or current across or through the head, respectively, is sensed by an amplifier which outputs useful signals representative of the information on the tape. For satisfactory operation of an MR tape head subsystem with a high signal-to-noise ratio (SNR), a high common mode rejection ratio (CMRR) and a high power supply rejection ratio (PSRR), the signal terminals of the head should have high impedance over the frequency range of operation to enable the terminals to pass a biasing signal without interference by the fast varying (high frequency) information signal from the head. Additionally, the input impedances of the signal terminals should be substantially the same despite different circuit components which may be interconnected with them.
The use of a two-terminal head has eliminated the biasing and impedance symmetry inherent with a three-terminal head. Moreover, a head can be subjected to varying operating conditions during its life, such as temperature fluctuations, and can undergo significant surface wear from contact of the moving tape with the head. For example, the resistance of a new MR head may be about 30 ohms. Due to wear, however, the resistance may slowly increase to about 120 ohms over the life of the head, a significant increase which can adversely affect the performance of a tape subsystem. And, process variations which occur during the fabrication of a batch of heads can result in small but significant variations in the initial resistance of different heads having the same nominal specifications. A single fixed-bias circuit design is not capable of compensating for the different and possibly changing biasing requirements imposed by these variations. Consequently, downtime for maintenance and early head replacement decreases the overall efficiency of a tape subsystem.